Reflective mask with high inspection contrast

ABSTRACT

A reflective mask may include an anti-reflective (AR) coating on an absorber layer to improve inspection contrast in an inspection system using deep ultraviolet (DUV) light. A silicon nitride (Si 3 N 4 ) AR coating may be used on a chromium (Cr) or tantalum nitride (TaN) absorber layer.

BACKGROUND

Masks used in lithography systems may be inspected by comparing light transmitted in the patterned (or “line”) region of the mask to the light transmitted in the non-patterned (or “space”) region. The defect detection sensitivity of the inspection system may depend on the difference in contrast between light transmitted in the two regions. Conventional transmissive optical masks may exhibit high inspection contrast since the light will either pass through the mask in the space region or will be blocked by the line region.

Low inspection contrast may present a problem in the inspection of reflective masks. Light absorber materials in the patterned region may typically reflect about 25% to about 45% of incident deep ultraviolet (DUV) light used for inspection purposes. The particular wavelength of the DUV inspection light may depend on the type of absorber, but may be in a range of about 150 nm to about 250 nm. The reflector region (e.g., the non-patterned region) may typically reflect about 60% of the DUV inspection light. As a result, the signal contrast between the patterned and non-patterned regions may be relatively low. In addition, there may be interference effects between the reflected light in the two regions due to the step height of the absorber. This interference effect may cause reflectance oscillation near the absorber edges, which may reduce the defect inspection sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a reflective mask.

FIG. 2 is a sectional view of blank reflective mask with a layer of resist material deposited during a mask fabrication process.

FIG. 3 is a sectional view of a patterned mask with defects.

FIG. 4 is a block diagram of a mask inspection system.

FIG. 5 is a sectional view of the mask of FIG. 3 with repaired defects.

FIG. 6 is a sectional view of a patterned mask with an exposed multilayer thin film (ML) reflector region.

FIG. 7 is a sectional view of an absorber layer with an anti-reflective (AR) coating.

FIG. 8 is a graph showing reflectivity responses of silicon nitride (S_(i)3N₄) AR coatings of different thicknesses on a chromium (Cr) absorber layer.

FIG. 9 is a graph showing reflectivity responses of S_(i)3N₄ AR coatings of different thicknesses on a tantalum nitride (TaN) absorber layer.

DETAILED DESCRIPTION

FIG. 1 shows a reflective mask 100. The mask may be used in an extreme ultraviolet (EUV) lithography system, which may use EUV radiation with wavelengths in a range of 10 nm to 14 nm to carry out projection imaging on a semiconductor wafer.

The mask may include multilayer thin films (ML) 105, a buffer layer 110, and an absorber layer 115 with an anti-reflective (AR) coating 120. The ML may include a large number of alternating layers of materials having dissimilar EUV optical constants. The alternating layers may be molybdenum (Mo) and silicon (Si), which may function well for wavelengths in the EUV spectrum.

The buffer layer 110 may be, e.g., ruthenium (Ru). The buffer layer may be used to repair defects in the absorber layer which may occur during pattern formation.

In an embodiment, the absorber layer 115 may include, e.g., chromium (Cr) or tantalum nitride (TaN), with a silicon nitride (Si₃N₄) AR coating 120. The absorber material in the absorber layer may be patterned such that absorber material is in the patterned (or “line”) region and the ML reflector is exposed in the non-patterned (or “space”) region.

A resist material 205 may be deposited on a blank reflective mask and then patterned and developed, as shown in FIG. 2. The absorber layer 115 may be etched to translate the pattern of the resist coating 205 to the absorber layer. However, defects may occur in the patterned absorber layer, such as the cavity 305 and the extra absorber defect 310 shown in FIG. 3.

The mask pattern may be inspected to determine if the pattern contains defects. The defects may then be repaired if necessary. FIG. 4 shows a reflective mask inspection system 400. The absorber layer pattern may be inspected by reflecting deep ultraviolet (DUV) radiation 405 off of the reflective mask 100 onto an image plane 410. The DUV inspection wavelength may be in a range of about 150 nm to about 250 nm. The reflected pattern may be captured by an image sensor 415. A machine vision system 420 may compared with the light received from the line and space regions with a desired pattern stored in a database 425. Any defects may then be repaired using, e.g. a filler material 500, as shown in FIG. 5. The buffer layer may then be removed to expose the ML reflector 105 in a space region 605, as shown in FIG. 6, and another inspection may be performed.

The accuracy of the inspection system 400 may depend on the degree of contrast between the light reflected in the line region (I(line)) and the light reflected in the space region (I(space)). Contrast may be determined using the following formula:

${Constrast} = \frac{{I({space})} - {I({line})}}{{I({space})} + {I({line})}}$

For the reflective mask shown in FIG. 1, I(line) may be about 25% to about 45% and I(space) may be about 60%. This may provide a contrast of only about 14% to about 30%.

For an AR coating on an absorbing material, the reflectance at the top surface may be given by the following equation:

$\begin{matrix} {R = \frac{{\left( {g_{1}^{2} + h_{1}^{2}} \right){\mathbb{e}}^{2\;\alpha_{1}}} + {\left( {g_{2}^{2} + h_{2}^{2}} \right){\mathbb{e}}^{{- 2}\;\alpha_{1}}} + {A\mspace{11mu}\cos\mspace{11mu} 2\;\gamma_{1}} + {B\mspace{11mu}\sin\mspace{11mu} 2\;\gamma_{1}}}{{\mathbb{e}}^{2\;\alpha_{1}} + {\left( {g_{1}^{2} + h_{1}^{2}} \right)\left( {g_{2}^{2} + h_{2}^{2}} \right){\mathbb{e}}^{{- 2}\;\alpha_{1}}} + {C\mspace{11mu}\cos\mspace{11mu} 2\;\gamma_{1}} + {D\mspace{11mu}\sin\mspace{11mu} 2\;\gamma_{1}}}} & (1) \end{matrix}$ where A=2(g ₁ g ₂ +h ₁ h ₂), B=2(g ₁ h ₂ −g ₂ h ₁), C=2(g ₁ g ₂ −h ₁ h ₂), D=2(g ₁ h ₂ +g ₂ h ₁)

$\begin{matrix} {{g_{1} = \frac{n_{0}^{2} - n_{1}^{2} - k_{1}^{2}}{\left( {n_{0} - n_{1}} \right)^{2} + k_{1}^{2}}},} & \; & \; & {{g_{2} = \frac{n_{1}^{2} - n_{2}^{2} + k_{1}^{2} - k_{2}^{2}}{\left( {n_{1} + n_{2}} \right)^{2} + \left( {k_{1} + k_{2}} \right)^{2}}},} \\ {{h_{1} = \frac{2n_{0}k_{1}}{\left( {n_{0} - n_{1}} \right)^{2} + k_{1}^{2}}},} & \; & \; & {h_{2} = \frac{2\left( {{n_{1}k_{2}} - {n_{2}k_{1}}} \right)}{\left( {n_{1} + n_{2}} \right)^{2} + \left( {k_{1} + k_{2}} \right)^{2}}} \\ {{\alpha_{1} = \frac{2\;\pi\; k_{1}d_{1}}{\lambda}},} & \; & \; & {\gamma_{1} = \frac{2\;\pi\; n_{1}d_{1}}{\lambda}} \end{matrix}$ In the above equations, d₁ is the thickness of the AR coating, n₁ is the index of refraction of the AR material, n₂ is the index of refraction of the absorber material, and k₁ are k₂ are optical constants (see FIG. 7). For equation (1), tantalum nitride (TaN) and chromium (Cr) absorber layers which are about 70 nm thick or thicker may be treated as bulk materials, e.g., incident radiation will not be reflected from the back surface of the absorber layer.

A minimum reflectivity may be obtained with an AR material having n₁, k₁, and a thickness d₁ which satisfy the following derivative conditions:

$\begin{matrix} {{\frac{\mathbb{d}R}{\mathbb{d}n_{1}} = 0},} & {{\frac{\mathbb{d}R}{\mathbb{d}k_{1}} = 0},} & {and} & {\frac{\mathbb{d}R}{\mathbb{d}d_{1}} = 0.} \end{matrix}$ Therefore, to obtain minimum reflectivity for a given absorber (n₂, k₂), only materials with n₁, k₁, and a thickness d₁ that satisfy the three derivative equations may yield the best anti-reflection effect.

The optics constants of silicon nitride (Si₃N₄) may match well with that of TaN and Cr. An Si₃N₄ AR coating has been found to provide a very low reflectance at DUV inspection wavelengths. The thickness of the Si₃N₄ AR coating 120 may be between about 120 Å to 250 Å for DUV inspection wavelengths of 150 nm to 250 nm. The thickness of the Si₃N₄ AR coating 120 may be selected to yield a near zero reflection at a desired DUV inspection wavelength.

FIG. 8 shows experimentally obtained reflectivity responses for Si₃N₄ AR coatings on a Cr absorber. Reflectivity responses were recorded for Si₃N₄ AR coatings with thicknesses of 120 Å 805, 150 Å 810, 180 Å 815, and 220 Å 820 and the case of no AR coating 825 over inspections wavelengths of 190 nm to 490 nm. As shown in FIG. 8, the reflectivity minimum moves from 250 nm to 235 nm when the AR thickness changes from 220 Å to 180 Å. The Si₃N₄ AR coating may be extended to inspection wavelengths below 190 nm by varying the AR coating thickness.

FIG. 9 shows experimentally obtained reflectivity responses for Si₃N₄ AR coatings on a TaN absorber. Reflectivity responses were recorded for Si₃N₄ AR coatings with thicknesses of 150 Å 905, 180 Å 910, and 260 Å 915 and the case of no AR coating 920 over inspections wavelengths of 190 nm to 490 nm. As shown in FIG. 9, the reflectivity minimum moves from 250 nm to 235 nm when the AR thickness changes from 260 Å to 180 Å. The Si₃N₄ AR coating may be extended to inspection wavelengths below 190 nm by varying the AR coating thickness.

With the Si₃N₄ AR coating on a TaN or Cr absorber layer, the inspection contrast may be reduced from about 14% to about 30% to greater than 96%. As a result of reduced reflection in the absorber region, many undesired effects during reflective EUV lithography (EUVL) mask inspection, such as interference effects, may be reduced or eliminated. Furthermore, the inspection sensitivity may be greatly enhanced.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. 

1. An apparatus comprising: a reflective mask having an inspection contrast of greater than about 30% for ultraviolet inspection light, the reflective mask comprising a reflector; and a plurality of absorber regions masking portions of the reflector, the absorber regions each including: absorber material comprising tantalum nitride; buffer material positioned between a first side of the absorber material and the reflector; and an anti-reflective coating comprising silicon nitride and positioned to cover a second side of the absorber material opposite the first side.
 2. The apparatus of claim 1, wherein the inspection contrast is greater than about 60%.
 3. The apparatus of claim 1, wherein the inspection contrast is greater than about 90%.
 4. The apparatus of claim 1, wherein the ultraviolet inspection light comprises light having a wavelength in a range of 150 nm to 250 nm.
 5. The apparatus of claim 1, wherein a thickness of the anti-reflective coating is between about 100 Å and 250 Å.
 6. The apparatus of claim 1, wherein the inspection light includes light of a wavelength λ, wherein the absorber material has an index of refraction n₀ and a thickness d₀, and wherein the anti-reflective coating comprises a material with an index of refraction equal to n₁, an absorption coefficient k₁, and a thickness d₁, and wherein d₁ is selected from possible thicknesses d so that a derivative of a reflectance R of the absorber regions with respect to the thickness d is about equal to zero.
 7. The apparatus of claim 1, further including a mask inspection tool, and wherein the reflective mask is positioned in the mask inspection tool.
 8. A method comprising: reflecting ultraviolet radiation off of a reflective mask having a first pattern, the reflective mask comprising a reflector and a plurality of absorber regions masking portions of the reflector, each of the plurality of absorber regions including a buffer layer, an absorber layer comprising tantalum nitride and covering the buffer layer, and an anti-reflective coating comprising silicon nitride and covering the absorber layer; and capturing a reflected pattern corresponding to the first pattern, the reflected pattern having an inspection contrast of greater than about 30%.
 9. The method of claim 8, wherein the inspection contrast is greater than about 60%.
 10. The method of claim 8, wherein the inspection contrast is greater than about 90%.
 11. The method of claim 8, further comprising detecting one or more mask defects by comparing the reflected pattern with an expected pattern.
 12. A reflective mask comprising: a pattern of absorber regions masking portions of a reflector, wherein each of the absorber regions comprises: protective material covering the reflector; absorber material comprising tantalum nitride and covering the protective material; and anti-reflective material comprising silicon nitride and covering the absorber material.
 13. The reflective mask of claim 12, wherein the absorber material has a thickness of about 50 nm or more.
 14. The reflective mask of claim 12, wherein the anti-reflective material has a thickness in a range of about 100 Å to 250 Å.
 15. The reflective mask of claim 12, wherein the absorber regions of the reflective mask have a minimum reflectivity at a wavelength between about 150 nm and 250 nm.
 16. The reflective mask of claim 12, wherein the absorber regions of the reflective mask has a minimum reflectivity less than about 0.05.
 17. A method comprising: forming a reflector on a substrate; forming a buffer layer to cover the reflector; forming an absorber layer to cover the buffer layer, the absorber layer comprising tantalum nitride; and forming an anti-reflective layer comprising silicon nitride to cover the absorber layer.
 18. The method of claim 17, wherein the anti-reflective layer has a thickness in a range of about 50 Å to about 250 Å. 